Linking voice and multimedia communication is playing a major role in the evolution and wider and more universal use of mobile cellular networks. While in the early years of mobile communications, speech calls were used independently of and uncorrelated with the then already existing possibility of exchanging short messages (SMS), users nowadays are increasingly interested in the possibility of rapidly switching between speech and text communication. There is even greater interest in so-called multimedia calls in which images or video sequences can be simultaneously presented to the respective party.
In the 3rd Generation Partnership Project (3GPP) mobile standard, a special service “Multimedia Service (UDI/RDI) with Fallback to Speech and Service Modification” has been described since Release 5 in the TS 23.172 specification. This service allows the mobile user to signal to the network at call setup that he or she would like to have the possibility of switching between the two services “speech” and “multimedia” (videotelephony) during the call.
Possible applications of this service include:    a) The user wishes to set up a speech call initially, but to have the option of subsequently switching to multimedia during the call in order to show something to the other party, it also being possible for the switchover to multimedia to be initiated by the other party. If one of the users so wishes, he or she can then switch back from multimedia to speech.    b) The user wishes to set up a multimedia call. If the network or the called user's mobile phone does not support a multimedia call or the called user only wishes to have a speech call instead of a multimedia call, by the signaling described in the TS 23.172 standard a speech call is set up instead of the multimedia call.    c) The user wishes to set up a multimedia call. If during the call the network cannot maintain the multimedia call, the network shall not allow the connection to be severed, but switch over in good time to a speech call (see TS 23.172, Section 4.2.5). This situation may arise, for example,            if the radio link becomes too poor for a multimedia call requiring a 64 kbit/s UDI radio bearer, but the transmission quality is still adequate for a 12.2 kbit/s radio bearer for speech (UDI=unrestricted digital information),        or if a handover takes place to another cell in which, because of a high traffic load, insufficient bandwidth is available for a 64 kbit/s UDI radio bearer,        or if a handover from one radio access network (UTRAN) to another (GERAN) takes place and the new radio access network does not generally support the multimedia call (see FIG. 1).        
According to TS 23.172, Section 4.2.5, in case c) the mobile switching center MSC that has initiated the changeover from multimedia to speech should offer the user a change back to multimedia if it subsequently determines during the conversation that a multimedia call would again be possible. The signaling for this purpose is described in TS 23.172.
In the standard, the network-initiated service change is currently only considered for case c). It would be desirable, however, for the network to offer the users a change to multimedia also in case b) if the multimedia call has not materialized only because no multimedia call was possible at one end or the other for network-internal reasons at the time of call setup (e.g. because one of the two users was located in a global system for mobile communications (GSM) cell at the time of call setup).
The standard also describes what should happen if, although the multimedia call is again possible at one end after a certain time, e.g. after the user has moved from a GSM to a UMTS cell, a multimedia call is now no longer possible at the other end of the connection, e.g. after the other user has moved from a UMTS to a GSM cell. (See FIG. 1: user A with mobile terminal MSA changes back from GERAN (2) to UTRAN (1), but user B with mobile terminal MSB has now changed from UTRAN (3) to GERAN (4); cf. also the description of the figures below.)
More generally, the problem is as follows: one of the two users would like to use a service 1 which, however, is only available in parts of the network or cannot be offered at all times because of a lack of certain resources. If the higher-grade service 1 cannot be provided by the network and the network instead sets up a connection for a service 2 or, if the connection for service 2 has already been set up, declines the service change from service 2 to service 1, the user request for service 1 should be stored in the network.
As soon as service 1 is available again in the network (e.g. because one of the users has moved to another radio cell), the change to service 1 should be offered to both users.
The user request should where possible be deleted from the network when the connection between the users is cleared down or if one of the two users does not accept the service change to service 1 proposed by the network (either by explicitly declining the change or by not responding to the offer and a monitoring timer then timing out in the network).
As described above, the standard currently only deals with case c) and even then offers only an incomplete solution for that scenario.
Quotation from TS 23.172, v 6.2.0, Section 4.2.5.2:    “The network initiated service change from speech to multimedia in lu mode is an optional feature. If supported the MSC initiating the service upgrade to multimedia shall use the following procedure. However a service change from speech to multimedia should not be initiated unless a network initiated service change from multimedia to speech had previously taken place during the same user session. . . . ”
However, the information as to whether a network-initiated service change from multimedia to speech had previously taken place is only available locally in the MSC which has implemented the change, as the signaling between the MSCs does not allow the other MSC to be informed why a service change is being requested (user- or network-initiated).
Consequently, the mechanism described by the standard fails if the upgrade to multimedia is possible at a particular point in time in MSC A, but is now no longer possible at the other end of the connection, in MSC B. As the signaling between MSC A and MSC B does not allow MSC A to be informed as to whether the service change from speech to multimedia has been declined by MSC B for network-internal reasons or because the user B did not want to change, MSC A will in this situation again delete the user request for a service change.